Serial digital data transmission is broadly divided between two classes: asynchronous and synchronous. Asynchronous transmission occurs when the intervals between input data transitions are nonuniform and independent of assigned channel timing characteristics. Synchronous transmission occurs when the intervals between data transitions are made uniform in accordance with assigned timing parameters. With asynchronous transmission the data transmission system adapts itself to the data. With synchronous transmission the data must be in a form compatible with fixed timing characteristics. Typical of asynchronous data is that generated by facsimile or other scanning apparatus. Asynchronous transmission is principally used at relatively low speeds, perhaps up to 1800 bits per second on voiceband channels.
Synchronous data is principally generated in data processors and may run into the thousands of serial bits per second in voiceband channels.
An intermediate type of data transmission is carried over from the Baudot code of printing telegraphy. Data are transmitted by character in the Baudot code, which uses five parallel hole positions on a tape to encode alphanumeric symbols. The code is transmitted serially with a start bit of fixed transition direction preceding each group of five message bits and a stop bit of oppositely directed transition. Modernization of the Baudot code has resulted in the nine-bit Extended Binary Coded Decimal Interchange Code (EBCDIC) and the ten-bit American Standard Code for Information Interchange (ASCII) codes. The additional bits make it possible to increase the number of available symbols in the code and to supply error-detecting parity bits. The newer codes include start and stop bits and are described generically as character asynchronous, that is, the time interval between characters is variable. However, the interval between bits within a character is largely synchronous, but not necessarily at the same rate as that of the transmission system.
It is an object of this invention to adapt the character-asynchronous data code format to the fully synchronous data transmission channel.
It is another object of this invention to provide a buffer between a character-asynchronous data format and a synchronous data transmission channel in such a way that the effective maximum input date rate can be exceeded by a moderate amount the synchronous channel data rate.
It is still another object of this invention to provide buffers between a character-asynchronous data format and a synchronous data transmission channel which adds or eliminates stop bits at the transmitter and restores a missing stop bit when necessary at the receiver to accommodate an input bit rate somewhat in excess of the synchronous rate.
It is yet another object of this invention to stretch steady space signals exceeding a character length to at least two character lengths without an intervening stop bit.